An exceptionally bright outburst from the HMXB XTE J1855-026
Off line analysis showed the presence of three flares separated by about the pulse period of 360 s.
The first flare started on 2010-03-14 at 19:02:27 (UTC) and lasted for about 130 s. We estimated for this flare a 20-40 keV peak luminosity of 0.9±0.1 Crab (we integrated in bins of 5 s). The average IBIS/ISGRI spectrum of the flare could be fitted by a power-law with index 2.5±0.3 and resulted in a 20-40 keV mean flux of (2.2±0.3) x 1e-9 erg/s/cm2 (0.29±0.04 Crab).
The second flare started on 2010-03-14 at 19:09:39 (UTC) and lasted about 55 s. The peak luminosity in the 20-40 keV energy band was of 0.7±0.1 Crab. The average spectrum of the flare could be fit by a power-law of index 2.2±0.4, and the corresponding flux was ~2e-9 erg/s/cm2 (~0.3 Crab; 20-40 keV).
The third flare started on 2010-03-14 at 19:15:16 and lasted for about 80 s with a peak flux of 0.30±0.09 mCrab (20-40 keV). Its average spectrum could be fit by a power-law of index 2.2±0.4, and the corresponding flux was ~1.2e-9 erg/s/cm2 (~0.2 Crab; 20-40 keV).
We also extracted the spectrum during the whole burst (from 2010-03-14 18:51 to 2010-03-14 19:46, UTC). This could be well fit by using a power-law of photon index 2.5±0.2 and the corresponding 20-40 keV X-ray flux was of (6.5±0.5) x 1e-10 erg/s/cm2 (~85 mCrab). Pulsations with a period of ~360 s were clearly detected during the entire event.
During the 5 hours before the outburst and the following 2.5 hours, the 20-40 keV flux of XTE J1855-026 was stable at about 30 mCrab. During the previous INTEGRAL observations performed between 2010-03-12 14:56 and 2010-03-13 04:33 in the direction of the source, XTE J1855-026 presented hour-scale variations in the 20-40 keV X-ray flux ranging from 10 to 60 mCrab.
The source was outside the JEM-X field of view during the burst.